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Downsized dwarf The distant frozen world of Eris, once considered to be the largest of the dwarf planets, may actually be the same size as Pluto.

The new measurement was obtained by a group of astronomers led by Professor Bruno Sicardy from the Paris Observatory, when the dwarf planet passed in front of a distant star - known as an occultation event.

This rare event was used to better estimate the dwarf planet's size and detect any trace of its atmosphere.

Reporting in the journal Nature, Sicardy and colleagues determine that Eris has a radius of about 1163 kilometres, which makes it roughly the same size as Pluto - estimated to be between 1150 and 1200 kilometres in radius - and far smaller than earlier estimates of around 1500 kilometres.

Sicardy and colleagues also determined Eris to have an average surface temperature of -243°C.

A distant world

Eris orbits the Sun three times as far out as Pluto, and up to 97 times as far out as the Earth. Owing to this great distance, measuring its size or detecting an atmosphere is difficult.

The new observations support previous predictions that Eris is basically spherical in shape. Its bright surface may be caused by a collapsed atmosphere, frozen solid by its cold environment.

However Sicardy and colleagues speculate that as Eris gets closer to the Sun in its elongated orbit, this solid coating may periodically sublimate - instantly change from a solid to a gas.

They say the same thing happens on Pluto, but Eris' atmosphere would be about 10,000 times thinner than Pluto's.

Eris' discovery in 2005, together with that of other similar-sized bodies played a part in Pluto's controversial demotion from its status as a planet to the new category of a dwarf planet in 2006.

Originally called "Xena", it was eventually formally named Eris after the goddess of strife and discord in Greek mythology.

Clever science

Planetary scientist Dr Brad Carter from the University of Southern Queensland, says by measuring the temporary dip in brightness of the star that Eris moved in front of it, Sicardy and colleagues were able to make some interesting measurements.

"But if you carefully time the duration of the occultation, how long Eris takes to pass in front of the star, from different parts of the Earth's surface you can build up a rough map or picture of both its size and shape."

"Knowing more about Eris and the other icy bodies out beyond Neptune will help us better understand how the solar system formed," says Carter.